In certain cellular communication systems, a stationary base station controls the transmission power level of a mobile station. In communication systems such as Advanced Mobile Phone Service (AMPS), Time Division Multiple Access (TDMA), and Global System for Mobile Communications (GSM), base stations instruct portable stations as to which one of several defined power steps should be used at each point in time. For example, the GSM specification defines fifteen uplink power control steps, each spaced 2 dBm apart and each having a tolerance of .+-.3 dBm.
A GSM base station broadcasts a power step command on a Broadcast Control Channel (BCCH) signal that is received by compatible mobile stations in its coverage area before any access attempt is made by a mobile station. Thus, when a portable telephone or other mobile station tries to contact the base station using a Random Access Channel (RACH) signal, it has instructions on which power step to use to initiate and maintain contact. A mobile station that has a maximum transmission power step lower than the power step stated in the BCCH signal simply uses the transmission power level corresponding to its maximum power step.
After a dedicated communication channel has been established between the base station and the mobile station, the base station sends further power step commands for that particular mobile station on a Slow Associated Control Channel (SACCH) signal. These further power step commands are determined based on reception level measurements performed by the base transceiver station and reception quality measurements taken and reported by the mobile station.
Base station power control allows the cellular system to improve overall spectral efficiency and also increase the battery life of the mobile station. In order to further increase the battery life of a mobile station, a portable telephone or other mobile station may transmit at a power other than the nominal power stated in the communication system specifications. For example, a GSM phone instructed to use power step 5 might transmit at 32 dBm instead of the nominal 33 dBm defined in the GSM specification. Although this lower power output increases the talk time of the phone, it may also cause a cellular telephone call to drop when a phone having a power output at the nominal 33 dBm would have successfully continued the call. If the mobile station is in a weak RF coverage area, the difference between the nominal power output and the increased talk time power output could cause the uplink signal to fail to reach the base station. Thus, there is a need for a power control system that extends the battery life of the mobile station, yet continues to obey base station power step commands and does not suffer from increased reliability problems in areas of weak RF signal coverage.